In this project we propose to study the mechanism and control of myofibrillar and mitochondrial assembly and degradation during the transition of the myocardium between two steady states. The proteins which turnover with heterogeneous rates will be studied. The main objectives are: 1. To determine whether the rate of synthesis (Rs) or the fractional rate turnover (kp) change during various phases of growth. Considering the heterogeneous nature of the turnover of proteins one of the rates necessarily must be changing during growth in order to maintain a constant ratio of individual proteins within the organelle. 2. To characterize myosin HC under conditions leading to altered ATPase activity. The putative isoenzymes will be characterized with respect to amino acid sequence (limited fragmentation) and immunological properties. The Rs and kp of separated HC will be measured during transition from one isoenzyme pattern to another one. To determine whether the relative fractional rates of turnover of structural proteins and those of the soluble proteins are changed during accelerated degradation. Proteins will be evaluated with respect to molecular weight, isoelectric point, and carbohydrate content. 4. The role of calcium-activated neutral protease (CaAp) in the turnover of myofibrils will be reexamined. The cellular origin of CaAp will be determined by immunofluorescence. Turnover of myosin HC and alpha-actinin will be determined in cultured heart cells after calcium influx is enhanced by an ionophore. The stability of the double hexagonal lattice of myofilaments will be examined after alpha-actinin removal. 5. To study the involvement of lysosomal proteases in the degradation of myofibrils. Attempts will be made to detect myosin or its high molecular-weight degradation products in the lysosomal fraction prepared from muscle.